1. Field of the Invention
This invention relates to a low iron loss grain-oriented electrical steel sheet suitable for cores of transformers and other electrical equipment.
2. Description of the Related Art
Grain-oriented electrical steel sheets used for cores of transformers and other electrical equipment require excellent magnetic characteristics, particularly low iron loss. This iron loss is usually represented as the sum of hysteresis loss and eddy current loss. In order to reduce iron loss, one or both of hysteresis loss and eddy current loss need to be reduced.
Hysteresis loss has sometimes been reduced to a large extent by highly orienting crystal grains of a steel sheet in a so-called Goss direction, that is, the {110}&lt;001&gt;direction, to enhance magnetic permeability. This has been done by using an inhibitor to inhibit the growth of crystal grains. On the other hand, eddy current loss has been reduced by increasing Si content in a steel sheet, or making the sheet thinner, or reducing the grain diameter of secondary recrystallized grains or forming a tension coating on a metal surface, or combinations of these.
Further, narrowing of magnetic domains artificially has reduced eddy current loss in recent years, and irradiating with laser rays (Japanese Examined Patent Publication No. 57-2252) and plasma flame (Japanese Unexamined Patent Publication No. 62-96617) have also been disclosed. In addition, for heat-proof domain-refining, grooves are formed on a steel sheet after secondary recrystallization by mechanical processing (Japanese Examined Patent Publication No. 50-35679) and linear notches orthogonal to the rolling direction are introduced before finishing annealing (Japanese Examined Patent Publication No. 3-39968). Further, disclosed in Japanese Unexamined Patent Publication No. 59-177349 is a method in which eddy current loss is reduced by appropriately controlling the inclination angle of crystals in the &lt;001&gt; direction from a rolling surface to reduce the widths of magnetic domains.
It has been intended, in conventional techniques, to integrate crystal grains into the Goss direction in order to reduce hysteresis loss and to reduce magnetic domain width in order to lower eddy current loss.
However, the conventional iron loss-reducing techniques suffer problems so that the iron loss has not yet sufficiently been reduced. The reasons include:
(1) iron loss increases due to non-uniform distribution of magnetic flux density originating in a difference (particularly a difference in the rolling plane) between grain directions of secondary recrystallized grains which are adjacent to each other in a direction orthogonal to the rolling direction (sometimes referred to as the rolling-orthogonal direction);
(2) when secondary recrystallized grains have a small diameter, the formation of magnetic poles originating in a difference between grain directions of the respective crystal grains reduces magnetic permeability and increases hysteresis loss; and
(3) as grain directions approach the Goss direction, the magnetic pole amount coming out on the steel sheet surface is lowered, and magnetic domain is broadened, so that eddy current loss becomes larger.
A method attempting to prevent degradation of iron loss has been disclosed in Japanese Unexamined Patent Publication No. 8-49045 by the present inventors. In that method the local change of magnetic flux density is made uniform over the whole steel sheet. A method involving controlling the composition of the coating, and the aspect ratio of secondary recrystallized grains, has been disclosed in Japanese Unexamined Patent Publication No. 8-288115 by the present inventors for practicing this technique. These methods can reduce uneven distribution of magnetic flux density originating in a difference between .alpha. angles (shearing angles in the [001] direction from the rolling direction in the rolling plane) of secondary crystallized grains adjacent in a rolling-orthogonal direction by inhibiting the growth of the secondary recrystallized grains in the rolling direction and accelerating the growth of secondary recrystallized grains in the rolling-orthogonal direction. However, when the secondary recrystallized grains in the rolling-orthogonal direction have large grain diameters, the growth rate of the secondary recrystallized grains in the rolling direction is likely to be accelerated as well. As a result, a suitable aspect ratio has not been attainable depending on the materials, and the iron loss has not sufficiently been reduced in some certain cases.
The artificial magnetic domain-refining method described above is effective against the problem (2) described above, but this magnetic domain-refining treatment brings about a degradation of magnetic permeability at the same time. Accordingly, it is difficult to reduce sufficiently a magnetic domain width without deteriorating magnetic permeability when depending only on conventional magnetic domain-refining techniques.
Further, with respect to the problem of item (3) described above, disclosed in Japanese Unexamined Patent Publication No. 6-89805 is a method in which fine grains having a diameter of 5 mm or less in addition to coarse secondary recrystallized grains are allowed to be present only in a prescribed number within a prescribed direction. However, this has not solved the problem of item (1) and therefore has faced the problem that when magnetic flux density is unevenly distributed in a plane of a sheet, due to a direction difference between secondary recrystallized grains adjacent in a rolling-orthogonal direction, the desired iron loss-reducing effect cannot be obtained.